Chronic pain by definition involves abnormal electrical spiking of neurons in the pain pathways: peripheral sensory neurons, spinal cord neurons, neurons in the pain matrix of the brain (e.g., somatosensory cortex, insular cortex, anterior cingulate cortex), and/or neurons in brainstem. Although firing of these neurons is modulated and governed by many different receptors, enzymes, and growth factors, in most neurons the fast upstroke of the electrical spike is produced by entry of sodium ions through voltage-gated sodium channels (Hille B, Ion Channels of Excitable Membranes. Sinauer Associates, Inc.: Sunderland Mass., 3rd Ed. 2001). There are nine different isoforms of voltage-gated sodium channel (Nav 1.1-Nav 1.9), and they have distinct expression patterns in tissues including neurons and cardiac and skeletal muscle (Goldin, A. L, “Resurgence of sodium channel research,” Ann Rev Physiol 63:871-894, 2001; Wood, J. N. and, Boorman, J. “Voltage-gated sodium channel blockers; target validation and therapeutic potential,” Curr. Top Med. Chem. 5:529-537, 2005). The various isoforms of voltage-gated sodium channels are:
Nav 1.1—found in the peripheral nervous system (PNS) and central nervous system (CNS), and believed associated with epilepsy, pain, seizures and neurodegeneration;
Nav 1.2—found in the CNS and believed associated with epilepsy and neurodegeneration;
Nav 1.3—found in the CNS and believed associated with pain;
Nav 1.4—found in the skeletal muscle and believed associated with myotonia;
Nav 1.5—found almost exclusively in cardiac tissue and believed to play a key role in cardiac action potential and propagation of electrical impulses, resulting in and associated with arrhythmia (Liu H, et al., Am. J. Pharmacogenomics, 3:173-179 (2003));
Nav 1.6—found in the CNS and PNS and believed associated with pain;
Nav 1.7—found in the PNS and believed associated with pain;
Nav 1.8—found in the PNS and believed associated with pain; and
Nav 1.9—found in the PNS and believed associated with pain.
Voltage-gated sodium channel isoforms have been divided into two subfamilies, based on those isoforms which are sensitive to blocking by tetrodotoxin (TTX-sensitive) and those which are resistant to blocking by tetrodotoxin (TTX-resistant). Nonselective sodium channel inhibitors such as lidocaine, mexiletine, and carbamazepine show clinical efficacy in chronic pain, including neuropathic pain, but they are limited in dose and in use, likely due to effects on sodium channels outside the pain pathway.
Recent evidence from several independent genetic studies has shown that the tetrodotoxin-sensitive voltage-gated sodium ion channel Nav 1.7 (SCN9A) is required to sense pain. Rare genetic forms of severe chronic pain, Primary Erythromelalgia and Paroxysmal Extreme Pain Disorder, result from mutations that increase the activity of Nav 1.7 (Fertleman C. R., Baker M. D., Parker K. A., Moffatt S., et al., “SCN9A mutations in paroxysmal extreme pain disorder: allelic variants underlie distinct channel defects and phenotypes,” Neuron 52:767-774, 2006; Yang Y., Wang Y., Li S, et al., “Mutations in SCN9A, encoding a sodium channel alpha subunit, in patients with primary erythermalgia,” J. Med. Genet. 41:171-174, 2004; Drenth J. P. H., te Morsche R. H. M., Guillet G., Taieb A., et al., “SCN9A mutations define primary erythermalgia as a neuropathic disorder of voltage gated sodium channels,” J Invest Dermatol 124:1333-1338). Conversely, two separate clinical studies have determined that the root cause of the genetic disorder Congenital Indifference to Pain (CIP) is a loss of function of Nav 1.7 via mutations that truncate the protein and destroy function (Cox J. J., Reimann F, Nicholas A. K., et al. “An SCN9A channelopathy causes congenital inability to experience pain,” Nature 444:894-898, 2006; Goldberg Y. P., MacFarlane J., MacDonald M. L., Thompson J., et al. “Loss-of-function mutations in the Nav1.7 gene underlie congenital indifference to pain in multiple human populations,” Clin Genet 71:311-319, 2007). The disorder is inherited in Mendelian recessive manner with 100% penetrance. The phenotype associated with CIP is extreme: affected individuals are reported to have experienced painless burns, childbirth, appendicitis, and bone fractures, as well as to have insensitivity to clinical measures of pain such as pinprick or tendon pressure. Yet sensory, motor, autonomic, and other measured functions are normal, with the only reported abnormality being anosmia (inability to smell). These studies indicate that among the many possible targets in the pain pathway, Nav 1.7 governs one or more control points critical for pain perception.
Nav 1.7 intervention may also be implicated in respiratory and respiratory tract diseases. In general, a cough results from various kinds of respiratory conditions and diseases. The cough reflex primarily protects the airway from possible harm via the clearance of foreign particulate and uninvited debris. Within the respiratory epithelium, nerve endings, sensing incoming irritants, transmit information regarding the presence of tussive stimuli to the brain, thereby inducing a cough reflex or cough response. When the cough progresses to a chronic cough, believed to be dry and unproductive, it is frequently associated with the development of lung damage, which is typically irreversible and commonly referred to as chronic pulmonary obstructive disease (CPOD). Such conditions (COPD) become a nuisance and progressively deteriorate one's quality of life. It has been shown that Nav 1.7 inhibitors have the potential to treat such respiratory and respiratory tract conditions, including post viral cough, viral cough and acute viral cough (PCT Publication WO2013006596). Accordingly, a therapeutic agent that inhibits Nav 1.7 should effectively treat pain and/or cough in humans.